SOIL SURFACE-CHARGE EVALUATION BY BACK-TITRATION .1. THEORY AND METHOD DEVELOPMENT

Until recently, surface charge chemistry of soils has been measured us ing two different approaches: ion adsorption and potentiometric titrat ion. The former provides a good measure of both anion-exchange capacit y (AEC) and cation-exchange capacity (CEC) as a function of pH, althou gh the procedure is time consuming. Potentiometric titration is more r apid, but the resulting curve overestimates the change in surface char ge with pH. The error is largely due to consumption of H+ and OH- by d issolution and hydrolysis reactions with dissolved ions instead of bei ng limited to protonation-deprotonation reactions of surface charge si tes. Our objective was to develop the theoretical aspects of the deter mination of surface charge using a back-titration method. After suspen sion in Ca(NO3)(2), a subsample of the soil is titrated slowly to pH 3 , and then back-titrated to pH 8 or 10. A second subsample is similarl y titrated to pH 3; the soil and the supernatant solution are separate d by centrifugation and filtering; only the supernatant solution is ba ck-titrated. The difference between the two titration curves represent s the consumption of OH- by surface reactions corrected for dissolutio n and other solution reactions. The proposed method provides a relativ ely rapid means of obtaining pH-dependent surface charge using potenti ometric titration while giving results comparable to those obtained by ion adsorption.